Artificial joints provide patients having arthritic or otherwise dysfunctional skeletal features with an alternative treatment for the chronic pain and discomfort often associated with such problems. Correction of the condition generally involves surgically replacing one or more of the natural components making up the joint with an artificial equivalent.
One of the more widely implemented artificial joints serves as a substitute for hips. A typical hip replacement system generally includes a femoral prosthesis implanted in the upper end of the femur when the femoral head requires replacement. The replacement is formed with a spherically shaped head and an elongated narrow neck extending from the head and connected to a stem which can be attached to the femur. The femoral head is pivotally nested within the socket of an acetabular cup. The cup includes a hemispherical base for mounting to the pelvis, and an outwardly opening socket to receive the femoral head. The prosthesis components are implanted during a surgical procedure well known to those skilled in the art.
While the typical hip replacement system described above provides a moderate range of mobility, the acetabular cup generally has limited clearance with respect to the neck of the femoral prosthesis. As a result, attempts by the patient to forcefully move the joint beyond the designed range of motion may cause the femoral head to pop out of the cup, resulting in dislocation that ultimately may require subsequent surgery for correction.
One attempt to expand the range of movement is disclosed in U.S. Pat. No. 5,387,244. The joint includes an acetabular cup with a bevelled edge for anchoring to the pelvis and a femoral prosthesis configured with a spherical head and a neck formed in lateral offset relation away from the medial side to the longitudinal axis of the femoral prosthesis. The neck includes a formed contact surface to complementally engage the bevelled edge of the cup to define a maximum degree of flexion.
While the design above may provide a relatively moderate range of mobility, the problem of dislocation remains unresolved. Dislocation typically occurs when the neck of the femoral component contacts the acetabular liner and rotates about that contact point. For the modified hip replacement system described above, the resultant contact point defined by the beveled edge and the contact surface occurs near the head center to create a fixed fulcrum that cooperates with the bulk of the prosthesis length to generate a relatively large moment. Under some circumstances, this moment is capable of dislodging or dislocating the head out of the cup. Moreover, continuous impact between these components can cause wear debris to accumulate in the joint.
Therefore, the need exists for a hip replacement system configured to minimize the occurrence of dislocation of the femoral component and the cup. The hip replacement of the present invention satisfies these needs.